Harmonics Concepts
Harmonics are voltages and currents at frequencies higher or lower than the nominal frequency (60Hz in the US, 50Hz in Europe). These frequencies are usually integer multiples of the nominal (‘fundamental’) frequency and are caused by devices that draw current in a non-linear fashion from the power source.
Typical nonlinear loads include rectifiers, Variable Frequency Drives (VFDs), and any other loads based on solid-state conversion. VFDs are a potential source of harmonics due to their fast current switching action at frequencies higher than the fundamental. Transformers and reactors may also become nonlinear elements in a power system during overvoltage/saturation conditions. Chargers for batteries (in the UPS system), for example, may also be a source of harmonics if these do not use the recent switch mode/chopper technology. Harmonics may also be received from other plants nearby, such as an aluminum smelter.
Power at harmonic frequencies does no useful work and causes equipment heating, interference and other undesirable effects. Electronics, including those onboard some VFDs, are particularly sensitive to high levels of harmonics. The presence of harmonics also decreases Power Factor, which is why engineers refer to ‘true’ PF in systems with harmonic distortion.
Figure 3.7- Harmonics-induced damage, (ABB Kurt Schipman ,‘Active Filter Savings’, 2008)
Harmonic pollution may also cause kWh meters to give faulty readings; these devices are important for monitoring energy consumption.
Harmonics are said to ‘distort’ the pure 60Hz sinusoidal waveform that carries useful power from the source. The summary effect of these waveforms yields the Total Harmonic Distortion (THD) measure.
Figure 3.8 – Graph of high harmonic content 50 Hz waveform, (ABB Kurt Schipman)
Figure 3.9 – Graph of harmonic current magnitudes of the above waveform, (ABB Kurt Schipman)
Although limits vary in different areas, the IEEE 519-1992 (USA), G5/4 (UK) standards limit THDV ≤ 5% and limit for each harmonic component. The worldwide standard IEC 61000-2-4 also limits THD. This limit is acceptable even for very sensitive loads. These standards also provide detailed harmonics mitigation design guidelines.
VFD-Induced Harmonics Concepts
Under ideal operating conditions, the current harmonics generated by a p-pulse line-commutated converter can be characterized by Ih=I1 / h and h=pn± 1 (characteristic harmonics) where n = 1, 2, ... and p is an integral multiple of six. In cases of non-ideal conditions (e.g. unbalanced converter input voltage, unequal commutation reactance,..), other harmonics can emerge: even, triple, odd non-characteristic and non-integral (inter-harmonics) harmonics belong to this group.
Harmonics Mitigation
It is considered best practice to reduce harmonics in the system at their point of origin. Installing filters near the harmonic sources, such as with VFDs, can effectively reduce harmonics. By using an individual passive filter for each VFD instead of a large centralized filter, one can also avoid the potential problem of over-correction at low loads. Also, the VFD with filter and motor with may then be moved elsewhere in the plant, to different buses without any re-engineering of a central system. The next best placement of filters is at the nearest switchboard or feeder switchgear.
Harmonics Mitigation Using Passive filters
The following section is adapted from material by ABB Kurt Schipman ‘Active Filter Savings’, 2008, and Dr. Amory Lovins in Competitek,1997.
For large, easily identifiable sources of harmonics, conventional passive filters designed to meet the demands of the actual application are the most cost efficient means of eliminating harmonics. These filters consist of capacitor banks with suitable tuning reactors and damping resistors. In addition to harmonics mitigation, these filters also perform PF correction because they act as a capacitor at the fundamental power frequency.
Some disadvantages of passive filters are listed below:
− Offer a low impedance path to harmonics
− Tuned below the first harmonic that exists in order to minimize resonance risk
− Filtering efficiency depends on network parameters, hence filtering performance cannot be guaranteed
− Danger for overloading due to load increase or background distortion, difficult to extend
− Multiple branches required for filtering more than one harmonic
− Multiple branches required for filtering multi-pulse arrangements
− Large space requirement and weight
− Always provide capacitive power: but AC drives do not require capacitive power
− Generators may not cope well with leading power factor
− Sizing rules not yet adapted to modern load types
− In LV applications, passive filters are used less and less
Figure 3.10 – Passive filter performance vs. frequency, (ABB, Schipman ‘Active Filter Savings’, 2008)
Harmonics Mitigation With Active Filters
For small and medium size loads, active filters, based on power electronic converters with high switching frequency, may be a more attractive solution. The basic concept is to put a switching inverter in parallel with the load that is injecting compensation currents at the harmonic frequencies. This approach is also known as a form of Static Synchronous Compensation (STATCOM).
Load Supply
Active Filter idistortion
Coupling system
icompensation
Load Supply
Active Filter Active Filter idistortion
idistortion
Coupling system
icompensation
icompensation
Figure 3.11 – Active filter with load schematic, (ABB, Schipman, 2008)
The advantages of parallel active filter topology are:
− Dimensioned for the compensation current only
− Cannot be overloaded irrespective of the harmonic load increase over time
− Does not directly affect the supply to the load
All commercial active filters on the LV market use parallel topology
Modern active filter technology is able to produce reactive power as well as mitigate, and in some cases, eliminate problems created by power system harmonics. See section on Power Factor and reactive power compensation for more details.
Active filters can perform the following Power Quality tasks:
− Harmonic filtration up to high order
− Reactive power compensation
− Load imbalance compensation
− Self-limiting
− Built-in network monitoring
Active filters work by measuring the physical voltage and current signal, analyzing them in real time in a high speed using digital signal processing (DSP), then creating the balancing power inverter control signals. These components are shown on the active filter schematic on the figure below.
The filter is programmable such that various degrees of filtering vs. reactive power compensation can be achieved. The filter’s control function may be either open loop or closed loop, but the trend is toward closed loop for these reasons (especially valid at higher frequencies):
− Directly measure and control harmonic current flowing to network
− Correction for system inaccuracies
− Can verify harmonics according to regulation directly
− Can be used for power factor targeting
− Simple CT connection using standard CTs
− Easy for future harmonic load extensions
− Appropriate for local & global compensation
Another trend in filter design is to have dedicated controllers for each harmonic order. This architecture, known as ‘frequency domain’, allows more precise targeting for each harmonic.
Figure 3.12 – Active filter schematic, (ABB, Schipman, 2008)
The highest amplitude harmonics are usually the 5th and 7th harmonics; these can be removed by using a 12-pulse uncontrolled diode bridge rectifier. A 24-pulse unit can be used for weaker networks or where more stringent THD requirements apply.
VFD-Induced Harmonics Mitigation
There are several approaches to mitigating harmonics induced by VFDs in the power supply system. The main methods are:
− Increasing the ‘resolution’ of the drive inverter by selecting those with a higher pulse count .
− Adding properly tuned filters on the supply line to the VFD.
− Newer technology active front-ends for the drive typically reduce harmonics induced on the supply side.
A comparison of the harmonic content between 6 and 12 pulse rectifiers is shown in the figure below:
Figure 3.13 – Harmonic distortion for 6 and 12 pulse rectifiers, (ABB Industrie AG, 2006)
Higher power applications (large MV drives) typically will use harmonics-free or high-pulse (18 high-pulse or higher) inverters to better distribute the load internally. An ideal 18-pulse drive will eliminate the 5/7/11/13th harmonics, but not the 17/19th (harmonics created are ‘pulse number +-1’). A harmonics analysis will reveal if these harmonics are problematic. For very large drive systems, 15MW or larger, a common solution might be a 12 (or 24)-pulse diode supply and passive filters on the line side or alternative a 36-pulse drive without a filter. A third approach is to specify a drive with active front end (Active Rectifier Unit: ARU) to control harmonics on the supply-side.
Case Example
Harmonics in a Pumping Cluster
The figure below shows line voltages & line currents at pumping cluster; the values for harmonic distortion were THDV = 12%, THDI=27%
Waveform event at 22/11/01 10:25:43.533
CHA Volts CHB Volts CHC Volts CHA Amps CHB Amps CHC Amps
VoltsAmps
10:25:43.72 10:25:43.73 10:25:43.74 10:25:43.75 10:25:43.76 10:25:43.77 10:25:43.78
-750
Figure 3.14– Line voltages and currents before filtering, (Schipman, 2008)
The following figure shows line voltage and currents with active filter, which improved the values for harmonic distortion to THDV = 2%, THDI=3%:
Waveform event at 22/11/01 10:41:55.533
CHA Volts CHB Volts CHC Volts CHA Amps CHB Amps CHC Amps
VoltsAmps
10:41:55.72 10:41:55.73 10:41:55.74 10:41:55.75 10:41:55.76 10:41:55.77 10:41:55.78
-750
Figure 3.15 – Line voltages and currents after filtering, (Schipman, 2008)